This invention relates to a plasma reformer for reforming a hydrocarbon fuel into a hydrogen-rich gas, and more particularly to a plasma reformer having an extended volume arc discharge.
Plasma reformers for converting a hydrocarbon fuel into a hydrogen-rich gas are well known. Such devices are often referred to as plasmatrons. In a plasmatron, a hydrocarbon fuel and air interact with an arc discharge which results in chemical reactions to reform the fuel.
U.S. Pat. No. 6,881,386 of which some of the inventors of the present application are co-inventors, teaches a plasmatron that efficiently uses electrical energy to produce a hydrogen-rich gas. In this patent, a high voltage, low current discharge works as a volumetric igniter of an air-fuel mixture providing a fast start of reactions of combustion, partial oxidation and fuel vaporization. The '386 patent includes two electrodes separated by an electrical insulator and disposed to create a gap so as to form a discharge volume for receiving the fuel-air mixture. A high voltage in the range 300V-60 kV is applied to the electrodes to create an arc between the electrodes. The arc is then stretched by air until extinction and immediately created again at a different location. The arc can not be stretched very far beyond the gap between the two electrodes because the plasma's electric field becomes unsustainable due to diminishing vorticity, current limitations and discharge shunting. A typical voltage within the arc is in the range of 1-2 kV that indicates a short arc length.
The relatively short residence time of the air-fuel mixture (at an O/C ratio of 1-1.8 for a partial oxidation reaction) in the discharge zone leads to the preferential development of a complete combustion reaction instead of a desired partial oxidation reaction. As a result, the hydrogen concentration in the hydrogen-rich gas and its heating value decreases.
To elaborate, because of the limited time that the air-fuel mixture (at an O/C ratio of 1-1.8) is in contact with a high temperature discharge, a portion of the fuel is completely combusted to CO2 and H2O instead of being reformed to CO and H2. The CO2 and H2O compounds can further react with the remaining hydrocarbons according to the reaction:CmHn+mH2O (CO2)→mCO+(n/2+m) H2 
This reaction is endothermic and its speed is much slower compared to complete combustion and partial oxidation (POX) reactions:Vcombustion>Vpox>VH2O(CO2) 
In order to preferentially achieve the POX reaction, it would be advantageous to increase the residence time of the air-fuel mixture (at an O/C ratio of 1-1.8) in the high temperature zone of the volumetric arc discharge. An extended time for the interaction of the air-fuel mixture with the high temperature (up to approximately 5,000 degrees C) plasma arc will create active radicals, improve mixing and provide continual initiation of the desired partial oxidation reaction instead of complete combustion. This extended time of the interaction can lead to complete non-catalytic partial oxidation of the hydrocarbon fuel. An alternative way to achieve the same goal is to treat the same air-fuel mixture with several consecutively positioned plasmatrons but that would be complicated and expensive.
An object of the present invention, therefore, is to increase residence time of the air-fuel mixture in a high temperature plasma zone leading to the efficient production of a hydrogen-rich gas.